Caroline Criado-Perez, the feminist campaigner, said she was relieved after two people pleaded guilty to sending her menacing tweets but added that dozens of other online abusers were still walking free.

John Nimmo, 25, from South Shields, and Isabella Sorley, 23, from Newcastle-upon-Tyne, pleaded guilty on Tuesday at Westminster magistrates court to abusing Criado-Perez online. Sorley was remanded in custody and Nimmo was bailed, both until 24 January, when they will be sentenced.

In a series of tweets, writing under a pseudonym, Sorley told Criado-Perez to "f*** off and die you worthless piece of crap", adding "go kill yourself" and "rape is the last of your worries". Another of her messages said: "I've only just got out of prison and would happily do more time to see you berried!"

In a separate set of messages sent to Criado-Perez, the court heard, Nimmo said to her, "shut up bitch", and "ya not that gd looking to rape u be fine", followed by "I will find you [smiley face]" and then the message "rape her nice ass".

The court heard that Nimmo also targeted the Walthamstow MP Stella Creasy with the message: "The things I cud do to u [smiley face]", calling her "dumb blond bitch".

Alison Morgan, prosecuting, said that Criado-Perez had received abusive messages from 86 Twitter accounts including those accounts attributed to Nimmo and Sorley.

Morgan told the court: "[She] has suffered life-changing psychological effects from the abuse which she received on Twitter. In particular the menacing nature of the tweets sent by both defendants caused her significant fear that they would find her and carry out their threats."

Nimmo said Criado-Perez was "not that good looking to rape". He used a number of accounts to goad his victims when they attempted to block his comments, saying police "would do nothing" about his tirade of abuse.

The court heard that he was a recluse whose only social interaction was online, on Facebook, Twitter and computer gaming. "The only time he leaves the house is to empty the bins," said Paul Kennedy, defending Nimmo. "He sits in the house with nothing to do, he claims benefits. He sits with a laptap, tablet and phone within touching distance at the end of the sofa."

Sorley, who said she was "bored" and "off her face" when she sent the messages, has 21 previous convictions, mostly for being drunk and disorderly.

The district judge Howard Riddle warned that she would "almost inevitably" be sent to prison when he passed sentence on 24 January.

Criado-Perez said she was happy about the guilty plea but disappointed that the perpetrators of the very worst abuse were not facing justice.

She said: "Today has been horrible, I've felt incredibly tense and have found it difficult to eat and breathe, so I'm happy they decided to plead guilty as we now do not have to go through the trial and it's over. But this was just the tip of the iceberg. These are just two people – there are many others who said the most awful, graphic things to me and they are not even under investigation. It feels very much like a pyrrhic victory."

She said other threats to her had spoken of mutilating her genitalia, stalking her outside her house, beating and gang-raping her. "It doesn't feel like justice has been done when those people are just going to get away with it. The two people who pleaded guilty today are just the ones the police could track – the others are walking free."

But she said she hoped the guilty pleas would encourage other women abused online to speak out. "From the look of my Twitter feed a lot of women are very happy about this , so that is a good way to look at it. If some justice is seen to be done then maybe that will give other women courage to come forward and hopefully make [abusers] think twice."

Sorley and Nimmo posted their tweets last July after the Bank of England governor, Mark Carney, said Jane Austen would replace Charles Darwin on the £10 note. Criado-Perez had begun her campaign after the bank announced in April that the social reformer Elizabeth Fry would be dropped from £5 notes in favour of Sir Winston Churchill, leaving no female figure on banknotes.

The Crown Prosecution Service previously said it would not pursue the prosecution of a separate suspect who allegedly sent offensive messages to Creasy.